RU2732261C1 - Method of producing high-strength sheet steel, characterized by high plasticity, deformability and weldability, and produced sheet steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: for producing sheet steel with applied coating, having given mechanical properties together with high weldability, especially high weldability when contact spot welding is used, method involves production of cold-rolled sheet steel containing the following, wt %: 0.15 ≤ C ≤ 0.23, 1.4 ≤ Mn ≤ 2.6, 0.6 ≤ Si ≤ 1.3, wherein C + Si/10 ≤ 0.30, 0.4 ≤ Al ≤ 1.0, wherein Al ≥ 6 (C+Mn/10) − 2.5, 0.010 ≤ Nb ≤ 0.035, 0.1 ≤ Mo ≤ 0.5, sheet annealing at temperature in range of 860–900 °C to obtain a structure consisting of at least 90 % of austenite and at least 2 % intercritical ferrite, hardening to temperature in range from Ms −10 °C to Ms −60 °C at speed Vc of more than 30 °C/s, heating to temperature RT in range from 410 °C to 470 °C during 60 to 130 s, coating is applied onto the sheet as a result of immersion into the melt and cooling to room temperature. Microstructure contains from 45 to 68 % of martensite, consisting of 85 to 95 % of martensite, subjected to redistribution of carbon and characterized by level of content of C, making, at most, 0.45 %, and fresh martensite; from 10 % to 15 % of residual austenite; from 2 % to 10 % of intercritical ferrite; from 20 % to 30 % of lower bainite.

EFFECT: invention discloses a method of producing high-strength sheet steel characterized by high plasticity, deformability and weldability, and produced sheet steel.

31 cl, 1 dwg, 11 tbl

Description

The present invention relates to a method for the production of high-strength steel sheet having high ductility and deformability together with high weldability, and to a sheet obtained using this method.

For the manufacture of various pieces of equipment, such as body parts and body panels for motor vehicles, it is known to use sheets made of DP (two-phase) steels or TRIP (martensitic plasticity) steels.

It is also known to use steels having a bainitic structure, free from precipitates of carbides, including retained austenite, containing about 0.2% C, about 2% Mn, about 1.7% Si and having a tensile yield strength of about 750 MPa. tensile strength of approximately 980 MPa, total elongation of approximately 8%. These sheets are produced in continuous annealing lines by cooling from an annealing temperature higher than the Ac ₃ transformation temperature up to a holding temperature above the Ms transformation temperature and keeping the sheet at that temperature for a predetermined time.

To reduce vehicle weight to improve fuel efficiency in a global environment, it is desirable to have sheets that have high yield and tensile strengths, together with good ductility and good deformability, and more specifically, good flangability. inner edges.

In this regard, it is desirable to have coated sheets having a tensile yield strength of at least 800 MPa, a TS tensile strength of at least 1180 MPa, a total TE elongation of at least 13% and a coefficient of a HER hole dispensing of at least 30%.

Tensile strength TS and total elongation TE are measured in accordance with ISO standard ISO 6892-1, published October 2009. As it must be emphasized, due to differences in measurement methods, in particular due to differences in the geometries of the samples used, the values of the total elongation TE according to the ISO standard are very different from the values of the total elongation measured according to JIS Z 2201- 05 standard, in particular, being smaller in comparison with them.

Hole expansion ratio HER is measured according to ISO standard 16630: 2009. Due to differences in measurement methods, the HER values of the hole expansion ratio according to ISO standard 16630: 2009 are very different from the hole expansion ratio λ according to JFS T 1001 (Japan Iron and Steel Federation standard) and cannot be compared with them. ...

However, in order to achieve this combination of strength and ductility, the addition of large amounts of C and Si is generally required. Such elements significantly reduce the weldability of steel, in particular the weldability when using resistance spot welding, leading to the presence of cracks in welds from resistance spot welding.

Therefore, the present invention aims to provide a coated steel sheet having the aforementioned mechanical features and properties together with high weldability, in particular high resistance spot weldability, and a method for producing the same.

For this purpose, the invention relates to a method for the production of coated steel sheet having a tensile yield point YS of at least 800 MPa, a tensile strength TS of at least 1180 MPa, total elongation, according to the document ISO standard 6892-1 of at least 13% and a HER expansion ratio according to ISO standard 16630: 2009 of at least 30%, where the method includes the following sequential steps:

- obtaining cold-rolled sheet steel made of steel, characterized by a chemical composition, including when expressed in% (wt.):

0.15% ≤ C ≤ 0.23%,

1.4% ≤ Mn ≤ 2.6%,

0.6% ≤ Si ≤ 1.3%,

while C + Si / 10 ≤ 0.30%,

0.4% ≤ Al ≤ 1.0%,

and Al ≥ 6 (C + Mn / 10) - 2.5%,

0.010% ≤ Nb ≤ 0.035%,

0.1% ≤ Mo ≤ 0.5%,

the remainder being Fe and inevitable impurities,

annealing the cold-rolled steel sheet at an annealing temperature T _A comprised between 860 ° C and 900 ° C, so as to obtain an annealed steel sheet having a structure of at least 90% austenite and at least 2% intercritical ferrite,

- hardening of the annealed steel sheet from the annealing temperature T _A up to the hardening temperature QT, comprised in the range from Ms - 10 ° C to Ms - 60 ° C, at a cooling rate of more than 30 ° C / s, to obtain a hardened sheet ,

- heating of the hardened sheet from the quenching temperature QT to the temperature of redistribution of carbon РТ, between 410 ° С and 470 ° С, and keeping the sheet at this temperature of redistribution of carbon РТ during the time of redistribution of carbon Pt, between 60 s and 130 from,

- coating sheet steel by immersion in a molten bath,

- cooling the steel sheet down to room temperature to obtain a coated steel sheet having a microstructure containing, expressed in surface fractional concentrations:

- from 45% to 68% of martensite, while the martensite consists of martensite subjected to redistribution of carbon, and fresh martensite, and from 85% to 95% of martensite is martensite, subject to redistribution of carbon, while the said martensite subject to redistribution of carbon is characterized by a C content of at most 0.45%,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite.

Preferably, the hardened sheet just before heating to the carbon redistribution temperature PT has a structure that, when expressed in surface fractional concentrations, is:

- from 2% to 10% intercritical ferrite,

- at least 10% retained austenite,

- at least 38% martensite and

- at least 15% lower bainite,

- at most 5% converted ferrite.

In accordance with one embodiment, the step for producing a cold rolled steel sheet comprises the following sequential steps:

- hot rolling of a semi-finished product made of said steel to obtain hot rolled steel sheet,

- rolling said hot-rolled sheet steel into a coil at a temperature Tc, comprised in the range from 400 ° C to 750 ° C,

- carrying out annealing in a chamber furnace at a temperature T _HBA , enclosed in the range from 500 ° C to 700 ° C, for a period of time enclosed in the range from 2 to 6 days,

- cold rolling said hot-rolled steel sheet to obtain said cold-rolled steel sheet.

Preferably, the cold rolled steel sheet is kept at an annealing temperature T _A for an annealing time t _A comprised between 80 s and 180 s.

According to one preferred embodiment, the annealing temperature T _A is at most 880 ° C. so that, after annealing, an annealed steel sheet is obtained having a structure of at least 90% austenite and more than 5% intercritical ferrite, wherein the microstructure of the coated steel sheet contains more than 5% intercritical ferrite.

Preferably, the coated steel sheet has a structure, expressed in surface proportions of:

- from 45% to 68% of martensite, while the martensite consists of martensite subjected to redistribution of carbon, and fresh martensite, and from 85% to 95% of martensite is martensite, subject to redistribution of carbon, while the said martensite subject to redistribution of carbon is characterized by a C content of at most 0.45%,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite,

- at most 5% converted ferrite.

Preferably, the average cooling rate Vc between the annealing temperature T _A and the quenching temperature QT is at least 50 ° C./s, the microstructure of the coated steel sheet being expressed in surface proportions from:

- from 45% to 68% of martensite, while the martensite consists of martensite subjected to redistribution of carbon, and fresh martensite, and from 85% to 95% of martensite is martensite, subject to redistribution of carbon, while the said martensite subject to redistribution of carbon is characterized by a C content of at most 0.45%,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite,

the coated steel sheet having a total elongation of at least 14%.

Even more preferably, the average cooling rate Vc between the annealing temperature T _A and the quenching temperature QT is at least 60 ° C / s, the total elongation being at least 14%, and the tensile strength being at least 1250 MPa ...

In one embodiment, the levels of Si and Al in the steel composition are such that 0.6% Si <1.0% and 0.7% Al 1.0%.

Preferably, the C content in the steel composition is such that 0.17% С C 0.21%.

Preferably, the level of Mn in the steel composition is such that 1.9% Mn 2.3%.

In one embodiment, the steel sheet is coated with a Zn or Zn alloy.

The invention also relates to a method for producing a resistance spot weld for at least two sheet steels, said method comprising:

- production of the first coated steel sheet using the method according to the invention,

- obtaining a second sheet steel, characterized by a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%,

- resistance spot welding between said first coated steel sheet and said second steel sheet.

For example, the second steel sheet is produced using the method according to the invention.

The invention furthermore relates to a coated steel sheet made from steel having a chemical composition including, expressed in% (mass):

0.15% ≤ C ≤ 0.23%,

1.4% ≤ Mn ≤ 2.6%,

0.6% ≤ Si ≤ 1.3%,

while C + Si / 10 ≤ 0.30%,

0.4% ≤ Al ≤ 1.0%,

and Al ≥ 6 (C + Mn / 10) - 2.5%,

0.010% ≤ Nb ≤ 0.035%,

0.1% ≤ Mo ≤ 0.5%,

the remainder being Fe and inevitable impurities,

wherein said coated steel sheet has a microstructure comprising, when expressed in surface fractional concentrations:

- from 45% to 68% of martensite, while the martensite consists of martensite subjected to redistribution of carbon, and fresh martensite, and from 85% to 95% of martensite is martensite, subject to redistribution of carbon, while the said martensite subject to redistribution of carbon is characterized by a C content of at most 0.45%,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite.

Preferably, retained austenite has an average C content comprised between 1.0% and 1.3%.

In one preferred embodiment, the microstructure of the coated steel sheet contains more than 5% intercritical ferrite.

In one embodiment, the levels of Si and Al in the steel composition are such that 0.6% Si <1.0% and 0.7% Al 1.0%.

In one embodiment, the level of C in the steel composition is such that 0.17% C 0.21%.

In one embodiment, the level of Mn in the steel composition is such that 1.9% Mn 2.3%.

In general, the coated steel sheet has a tensile yield strength of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation in accordance with ISO standard 6892-1 of at least 13%, and an opening ratio according to ISO 16630: 2009 HER of at least 30%.

In one embodiment, the coated steel sheet is coated with a Zn or Zn alloy, the coating being the result of being coated at a temperature of less than 480 ° C.

Preferably, the coated steel sheet has a structure, expressed in surface proportions of:

- from 45% to 68% of martensite, while the martensite consists of martensite subjected to redistribution of carbon, and fresh martensite, and from 85% to 95% of martensite is martensite, subject to redistribution of carbon, while the said martensite subject to redistribution of carbon is characterized by a C content of at most 0.45%,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite,

- at most 5% converted ferrite.

Preferably, the microstructure, expressed in surface fractional concentrations, consists of:

- from 45% to 68% of martensite, while the martensite consists of martensite subjected to redistribution of carbon, and fresh martensite, and from 85% to 95% of martensite is martensite, subject to redistribution of carbon, while the said martensite subject to redistribution of carbon is characterized by a C content of at most 0.45%,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite.

In this embodiment, the overall elongation is generally at least 14%.

In accordance with one embodiment, the tensile strength is at least 1250 MPa and the total elongation is at least 14%.

The invention further relates to a welded structure comprising at least ten resistance spot welds for at least a first steel plate and a second steel plate, where the first steel plate is a coated steel plate according to the invention and the second sheet steel is characterized by a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%, and where the average number of cracks per one weld from resistance spot welding is less than 6.

In one embodiment, the second steel sheet is a coated steel sheet according to the invention.

Preferably, the welded structure is a welded structure of a first steel plate and a second steel plate (i.e., only two steel plates), with an average number of cracks having a depth greater than 100 microns, among at least ten welds from resistance spot welding is less than 0.1.

In the case of a welded structure being a welded structure of a first steel plate, a second steel plate, and a third steel plate, the third steel plate is characterized by a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%, the average number of cracks having a depth of more than 100 microns, among at least ten welds from resistance spot welding, is generally less than 4.

The invention also relates to a method for manufacturing a welded structure according to the invention, comprising:

- obtaining the first sheet steel and the second sheet steel,

- partial overlap of the first sheet steel and the second sheet steel,

- application of a force between 3.5 and 5 kN when using electrodes located perpendicular to the superimposed sheets,

- resistance spot welding for the first and second sheet steels in order to produce at least 10 resistance spot welds, the intensity being comprised in the range from Imax to 1.1 * Imax, where Imax is the intensity at which displacement of liquid metal by resistance spot welding is observed for said first coated steel sheet and said second steel sheet.

The invention also relates to the use of the coated steel sheet produced according to the invention or the coated steel sheet according to the invention for the manufacture of structural parts for automobile vehicles.

The invention also relates to the use of a resistance spot weld made according to the invention or a welded structure according to the invention for the manufacture of structural parts for motor vehicles.

The invention will now be described in detail, but without imposing limitations, with reference to the accompanying figure, which shows a photomicrograph for the steel according to the invention.

The composition of the steel according to the invention contains, expressed in mass percent:

- from 0.15% to 0.23% carbon to provide satisfactory strength and improve the stability of retained austenite, which is necessary to obtain sufficient elongation. Preferably, the carbon content is greater than or equal to 0.17% and / or less than or equal to 0.21%. In the case of an excessively high carbon content, the hot rolled sheet will be too hard for cold rolling, and the weldability, especially the resistance spot weldability, will be insufficient. When the carbon content is less than 0.15%, the tensile strength will not reach 1180 MPa.

- from 1.4% to 2.6% manganese. The minimum is determined to obtain sufficient hardenability to obtain a microstructure containing at least 45% martensite and at least 85% redistributed martensite in martensite, and a tensile strength of at least 1180 MPa. The maximum is determined to avoid the occurrence of segregation problems, which are detrimental from a plasticity point of view. Preferably, the manganese content is greater than or equal to 1.9% and / or less than or equal to 2.3%.

- from 0.6% to 1.3% silicon and from 0.4% to 1.0% aluminum. In the steel of the invention, both Si and Al were added as austenite stabilizers. In particular, Si and Al retard the formation of carbides in the case of cooling the steel sheet at a certain temperature so as to obtain incomplete martensitic transformation, and immediately reheating and keeping it at PT temperature, during which carbon is redistributed from martensite to austenite. If Si and Al are added in sufficient amounts, the redistribution of carbon will occur in the absence of significant formation of precipitates of carbides. In addition, Si provides solid solution hardening and improves the expansion ratio.

However, the Si content should be limited to 1.3% in order to avoid the formation of silicon oxides on the surface of the sheet, which would be detrimental from a coating standpoint.

Most of all, as discovered by the inventors, in the case of Si / 10> 0.30% - C (while the levels of the percentage of Si and C are expressed in mass percent), due to the presence of LMO embrittlement (the phenomenon of liquid metal embrittlement), silicon will be harmful for resistance spot welding galvanized sheets. The occurrence of embrittlement of LMO causes the appearance of cracks at the grain boundaries in the heat-affected zones and in the welded metal of welded joints. Therefore, the value (C + Si / 10) must be kept less than or equal to 0.30%. Preferably, the Si content is at most 1.0%.

In addition, Al must be added at a level of at least 0.4%, so as to achieve sufficient stabilization of the austenite. However, the Al content is limited to 1.0% in order to prevent the Ac3 transformation temperature from increasing, which would imply an increased cost in the case of heating at a high temperature to austenitize the steel sheet in the annealing step.

In addition, Al has a beneficial effect by reducing the susceptibility to embrittlement of LMOs at high levels of C and / or Mn. Thus, the Al content is such that Al ≥ 6 (C + Mn / 10) - 2.5%. Preferably, the Al content is at least 0.7%.

- from 0.010% to 0.035% niobium in order to refine the austenite grains during hot rolling and obtain precipitation hardening during the final heat treatment. The Nb content in the range of 0.010% to 0.035% makes it possible to obtain satisfactory levels of tensile yield strength and elongation, in particular a tensile yield point of at least 800 MPa.

- from 0.1% to 0.5% molybdenum to increase hardenability and stabilize retained austenite in order to greatly reduce the decomposition of austenite during carbon redistribution. Preferably, the Mo content is at least 0.20%.

The remainder is iron and residual elements from the steelmaking process. In this respect, Ni, Cr, Cu, Ti, V, B, S, P and N are at least considered as residual elements, which are unavoidable impurities. Therefore, their levels in terms of mass percent are less than 0.05% for Ni, 0.01% for Cr, 0.03% for Cu, 0.007% for V, 2 ppm. for B, 0.005% for S, 0.02% for P, and 0.010% for N. The Ti content is limited to 0.05%, since above these values, large precipitates of carbonitrides will form mainly at the liquid phase stage, and deformability sheet steel will deteriorate, making the 13% target for total elongation more difficult to achieve.

In the case of the coated steel sheets of the invention, in particular those coated with Zn or Zn alloy coatings, the weldability using resistance spot welding e may be affected by the LMO embrittlement phenomenon (liquid metal embrittlement).

The susceptibility of a particular steel to this phenomenon can be assessed using a tensile test conducted at a high temperature. In particular, this hot tensile test can be carried out using a Gleeble RPI thermosimulator, a device known per se in the art.

This test, called the “Gleeble LME test”, is described as follows:

- Samples of coated sheets having a thickness in the range of 0.7 mm to 3 mm are subjected to tensile tests at high temperature in order to determine what is the minimum critical displacement for which cracking occurs in the vicinity of the weld zone. Samples that are cut into the sheet have a calibrated zone that is 10 mm long and 10 mm wide and heads that are 40 mm long and 30 mm wide, with a radius of curvature between the heads and the calibrated portion of 5 mm. ...

- tensile tests at high temperature are carried out as a result of rapid heating (1000 ° C / s) of each sample, keeping the sample at a predetermined temperature and implementing a predetermined relative elongation or displacement for the heated sample, after which the sample is cooled in air, while the relative elongation or displacement is retained. After cooling, the samples are monitored to determine the presence or absence of LMO embrittlement cracking. By definition, a sample will have a crack if at least one crack of at least 2 mm is formed on the sample.

- tests are carried out at many predetermined temperatures, such as 700 ° C, 750 ° C, 800 ° C, 850 ° C, 900 ° C and 950 ° C, and at elongations or displacements of 0.5 mm, 0.75 mm, 1mm, 1.25mm, 1.5mm, 1.75mm, 2mm and so on; Elongations or displacements are the elongations or displacements of the clamps that hold the samples on the Gleeble.

- give the critical displacement for the initiation of cracking and for the temperature range under consideration determine the minimum critical displacement, that is, the minimum displacement for which cracking occurs.

As is generally believed, in the case of a minimum critical displacement of less than 1.5 mm at a temperature in the range from 700 ° C to 800 ° C, the likelihood of LMO embrittlement in resistance spot welding will be high, and in the case of a minimum critical displacement of at least 1.5 mm, the likelihood of observing many LMO embrittlement cracks in resistance spot welding will be low.

In this regard, as found by the inventors for steels of the invention, such that the (C + Si / 10) value is less than or equal to 0.30% and the Al content is greater than or equal to 6 (C + Mn / 10) - 2, 5%, the minimum critical displacement is at least 1.5 mm, and in the case of a (C + Si / 10) value of more than 0.30% and / or an Al content of less than 6 (C + Mn / 10) - 2.5%, the minimum critical displacement will be less than 1.5 mm and even less than 1 mm.

As examples, the Gleeble LME tests were carried out using steels characterized by the following compositions:

S1: C = 0.226%, Mn = 2.01%, Si = 0.716%, Al = 0.802%.

S2: C = 0.204%, Mn = 2.07%, Si = 1.44%, Al = 0.033%.

For S1 C + Si / 10 = 0.2976% and the minimum critical displacement is 2.25 mm.

For S2 C + Si / 10 = 0.4412% and the minimum critical displacement is 0.9 mm.

Another method for evaluating resistance spot weldability for coated sheets is the “flash resistance spot welding test with embrittlement of LMOs”, which makes it possible to determine the likelihood of cracked welds in a significant number of resistance spot welds, for example , in the industrial production of products including parts that are assembled together using resistance spot welding, such as, for example, car bodies.

This “flash resistance spot welding test with LMO embrittlement” is derived from a resistance spot welding electrode life test in which a plurality of resistance spot welds, for example 30, are created on three sheets superimposed on each other: tested and two load-bearing sheets made of galvanized mild sheets, for example, grade DX54D + Z according to EN 10346. The sheet thickness is 1.6 mm and the resistance spot welds are made according to ISO Standard 18278-2 for assemblies of dissimilar parts. The parameters are:

- diameter of the working end of the electrode: 8 mm,

- welding force: 4.5 kN,

- welding time: 3 pulses of 180 ms, separated by periods of 40 ms (cooling times),

- holding time: 400 ms.

For this test, the specimens are cut and polished to determine the possible occurrence of cracks in the weld seams from resistance spot welding. After that, the resistance spot welds are pickled using picric acid and observed using a microscope, for example, at a magnification of 200x, in order to determine the number of cracks in each observed resistance spot weld and the total value of the crack length in each resistance spot weld.

For examples S1 and S2, the proportions of the number of cracks for each resistance spot weld are as follows:

- S1: Gleeble LME test ≥ 1.5 mm, 80% of resistance spot welds have less than 10 cracks and 0% have 20 or more cracks,

- S2: Gleeble LME test <1.5 mm, only 40% of resistance spot welds have less than 10 cracks and 30% have 20 or more cracks.

When considering the average number of cracks in each resistance spot weld, the results would be as follows:

- S1: the average number of cracks in each weld from resistance spot welding is 5,

- S2: The average number of cracks in each resistance spot weld is 10.

A hot rolled sheet having a thickness in the range of 2 to 5 mm can be produced in a known manner from the aforementioned inventive steel composition.

As one example, the reheating temperature before rolling may be in the range of 1200 ° C to 1280 ° C, preferably about 1250 ° C, the finish rolling temperature is preferably in the range of Ar3 to 950 ° C, and is preferably more than 850 ° C, and rolling is carried out at a temperature preferably comprised between 400 ° C and 750 ° C. Preferably, for Si> 1.0%, the roll-off temperature will be less than or equal to 550 ° C.

After rolling into a roll, the sheet has a ferrite-bainite-martensitic or ferrite-perlite-bainite structure.

After being coiled, the sheet is preferably subjected to annealing in a chamber furnace in order to reduce the hardness of the hot rolled steel sheet and therefore improve the cold rollability of the hot rolled steel sheet.

For example, hot rolled steel sheet is annealed in a chamber furnace at a temperature in the range of 500 ° C to 700 ° C, for example, 550 ° C to 650 ° C, for a period of time in the range of 2 to 6 days, preferably 3 to 5 days. This time includes heating to the annealing temperature in the chamber furnace and cooling from the annealing temperature in the chamber furnace to ambient temperature.

After annealing in a chamber furnace, if this is done, the cold rolled steel sheet will have a ferrite-bainite-tempered martensite structure.

The hot rolled and optionally box furnace annealed steel sheet is optionally pickled, then cold rolled to obtain cold rolled steel sheet having a thickness in the range of 0.7 mm to 3 mm, for example in the range of 0.8 to 2 mm.

Thereafter, the cold rolled steel sheet is subjected to a heat treatment, preferably in an integrated continuous annealing and hot dip plating line.

Heat treatment and coating include the steps:

- annealing of cold-rolled steel sheet at an annealing temperature T _A , between 860 ° C and 900 ° C, while the annealing temperature T _A is such that at the end of the annealing stage, the steel has a structure consisting of austenite and intercritical ferrite, and the fractional the concentration of austenite is at least 90%, and the fractional concentration of intercritical ferrite is at least 2%. Thus, the annealing is carried out at a temperature lower than Ac3, with Ac3 being the temperature of completion of the transformation to austenite during the heating step. In the case of an annealing temperature T _{A of} less than 860 ° C, the final structure (i.e., after heat treatment and coating) will achieve insufficient proportional concentrations of martensite and retained austenite so that the target tensile strength is not achieved and total elongation. In the case of an annealing temperature T _{A of} more than 900 ° C, the final structure will be characterized by an insufficient fractional concentration of intercritical ferrite such that a total elongation of at least 13% is not achieved. Preferably, the annealing temperature is at most 880 ° C. so as to achieve a structure consisting of at least 90% austenite and more than 5% intercritical ferrite upon completion of the annealing step.

The sheet is kept at the annealing temperature, that is, kept in the range from T _A - 5 ° C to T _A + 5 ° C, during the annealing time t _A , preferably comprised between 80 s and 180 s. Preferably, the annealing time t _A is between 85 s and 136 s.

- immediately after annealing, the quenching of the annealed steel sheet as a result of cooling it from the annealing temperature T _A to the quenching temperature QT, lower than the transformation temperature Ms of austenite obtained after annealing, at a cooling rate high enough to avoid the formation of upper and granular bainite and avoid or limiting the formation of ferrite.

The average cooling rate from the annealing temperature T _A to the quenching temperature QT is strictly more than 30 ° C / s, preferably more than 50 ° C / s, even more preferably more than 60 ° C / s.

Indeed, in the case of a cooling rate of 30 ° C / s or less, the target yield and tensile strengths will not be achieved. A cooling rate of more than 30 ° C / s makes it possible to restrict the formation of ferrite after cooling with a fractional concentration of less than 5%.

The cooling rate of at least 50 ° C / s makes it possible to suppress the formation of ferrite after cooling. Thus, a cooling rate of at least 50 ° C / s ensures that the structure of the sheet after quenching, and therefore the final structure of the coated steel sheet, does not contain any ferrite resulting from the transformation of austenite to ferrite. A cooling rate of at least 50 ° C / s makes it possible to achieve a total elongation of at least 14% in combination with the target tensile yield strength, tensile strength and expansion ratio.

A cooling rate of at least 60 ° C / s makes it possible to achieve a total elongation of at least 14% and a tensile strength of at least 1250 MPa.

The hardening temperature is in the range from Ms - 60 ° C to Ms - 10 ° C. For each particular steel composition and structure, those skilled in the art know how to determine the transformation temperature Ms of austenite remaining after annealing. In the case of a quenching temperature QT of less than Ms - 60 ° C, the fractional concentration of tempered martensite created after quenching will be too high to stabilize a sufficient amount of retained austenite of more than 10% in the final structure so that the total the relative elongation will not reach 13%. In addition, in the case of a quenching temperature QT greater than Ms - 10 ° C, the fractional concentration of redistributed martensite in the final structure will be too low to obtain the desired tensile strength.

The structure of the steel at the hardening temperature QT preferably contains:

- from 2% to 10%, and preferably more than 5%, of intercritical ferrite,

- at least 10% retained austenite,

- at least 38% martensite and

- at least 15% lower bainite.

Intercritical ferrite is the result of annealing at a temperature lower than Ac3. Intercritical ferrite is different from the ferrite that would be formed after annealing, hereinafter referred to as "converted ferrite" and resulting from the conversion of austenite to ferrite. Such converted ferrite could, for example, occur during cooling from the annealing temperature T _A to the quenching temperature QT if the cooling rate is insufficient to prevent such formation (ie, 30 ° C / s or less). In particular, in contrast to converted ferrite, intercritical ferrite is polygonal. In addition, the converted ferrite is enriched in carbon and manganese, that is, has levels of carbon and manganese that are higher than the levels of carbon and manganese in the intercritical ferrite. Therefore, intercritical ferrite and converted ferrite can be differentiated by obtaining, at the time of observation, a micrograph using an AEP-TEM microscope using secondary electrons, after pickling using metabisulfite. In such a photomicrograph, intercritical ferrite has a medium gray color, while converted ferrite has a dark gray color due to its higher carbon and manganese levels.

The structure at the quenching temperature QT may contain converted ferrite, however, at a fractional concentration of at most 5%, and in general at most 2%.

Thus, the structure of steel at the hardening temperature generally consists of:

- from 2% to 10%, and preferably more than 5%, of intercritical ferrite,

- at least 10% retained austenite,

- at least 38% martensite,

- at least 15% lower bainite and

- at most 5% converted ferrite.

In addition, in the case of a cooling rate to the QT quenching temperature of at least 50 ° C./s, the structure at the QT quenching temperature will not contain any converted ferrite. Thus, the structure at the quenching temperature QT is expressed in surface fractional concentrations from:

- from 2% to 10%, and preferably more than 5%, of intercritical ferrite,

- at least 10% retained austenite,

- at least 38% martensite and

- at least 15% lower bainite.

the quenched sheet is optionally held at a quenching temperature QT for a holding time ranging from 2 s to 8 s, preferably from 3 s to 7 s.

- after that, the sheet is reheated from the quenching temperature up to the carbon redistribution temperature PT, which is in the range from 410 ° C to 470 ° C, and is kept at the carbon redistribution temperature PT during the time of carbon redistribution Pt, which is in the range from 60 s to 130 from. During this stage of carbon redistribution, carbon is redistributed, that is, diffuses, from martensite to austenite, which is thus enriched in carbon.

In the case of a carbon redistribution temperature PT of more than 470 ° C or less than 410 ° C, the elongation of the final product will be insufficient.

In the case of a carbon redistribution time of less than 60 s, there will be insufficient carbon redistribution from martensite to austenite, such that the carbon content in the martensite will be excessively high and the carbon content in the retained austenite will be excessively low. As a result, the elongation of the final product will be unsatisfactory.

- the sheet is coated by immersion in the melt immediately after the stage of preserving the sheet at the temperature of carbon redistribution RT. Hot dip plating can be, for example, zinc plating, but it is possible to apply all hot dip metal coatings, provided that the temperatures that the sheet is brought to during coating are kept at less than 480 ° C. FROM. In the case of galvanizing the sheet, this will be done under normal conditions, for example by passing through a Zn bath at a temperature in the range of 430 to 480 ° C. The steel according to the invention can be galvanized using Zn or a Zn alloy, like, for example, zinc-magnesium or zinc-magnesium-aluminum.

- immediately after the hot-dip plating step, the coated steel sheet is cooled to room temperature at a cooling rate preferably greater than 1 ° C / s, for example in the range from 2 ° C / s to 20 ° C /from.

This heat treatment and coating makes it possible to obtain the final structure (i.e., after carbon redistribution, coating by immersion in the melt and cooling to room temperature), which, when expressed in fractional concentrations:

- from 45% to 68% martensite,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite.

Martensite consists of carbon redistributed martensite and fresh martensite. Between 85% and 95% of the martensite is carbon redistributed martensite, with the remainder, that is, 5% to 15%, fresh martensite.

In contrast to the tempered martensite, which would be obtained using a heat treatment including annealing, quenching and tempering, the redistributed martensite in the steel of the invention has a C content of at most 0.45%. This level is the result of the redistribution of carbon from martensite to austenite during the carbon redistribution step.

In contrast, fresh martensite, which results from the conversion of carbon-rich austenite to martensite after the carbon redistribution step, has a C content of at least 0.9%, and generally less than 1.2%.

The fractional concentration of fresh martensite in relation to martensite is in the range from 5% to 15%, so that the fractional concentration of fresh martensite in relation to the total structure is at most 10%. Indeed, a fractional concentration of fresh martensite of more than 10% would result in a hole expansion ratio HER according to Standard ISO 16630: 2009 of less than 30%.

The fractional concentration of retained austenite of at least 10%, together with the fractional concentration of intercritical ferrite of at least 2%, makes it possible to obtain a total relative elongation of at least 13%, while the total relative elongation is measured in accordance with the document ISO Standard 6892-1.

In addition, this treatment makes it possible to obtain an increased level of C content in retained austenite, which is at least 1.0% and up to 1.3%. This increased C content stabilizes the retained austenite and contributes to achieving a total elongation of at least 13%.

Ferrite in the structure is intercritical ferrite, that is, ferrite that is the result of annealing at a temperature lower than Ac3.

The fractional concentration of intercritical ferrite is preferably in the range from 5% (excluding 5%) to 10%.

The coated steel sheet according to the invention may contain converted ferrite, however, at a fractional concentration of at most 5%, generally at most 2%. Thus, the structure of the coated steel sheet according to the invention consists of:

- from 45% to 68% martensite,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite,

at most 5%, and preferably at most 2% converted ferrite.

Preferably, the structure does not contain any converted ferrite.

Thus, the structure of the coated steel sheet preferably consists of:

- from 45% to 68% martensite,

- from 10% to 15% of retained austenite,

- from 2% to 10% intercritical ferrite,

- from 20% to 30% lower bainite.

Signs of microstructure are determined, for example, by observing the microstructure using a scanning electron microscope with a field emission gun ("AEP-SEM") at a magnification greater than 5000x, in combination with a device for diffraction of back-scattered electrons ("DRE") and transmission electron microscopy (TEM).

By using this heat treatment, steel sheets can be obtained having a tensile yield strength YS of at least 800 MPa, a tensile strength TS of at least 1180 MPa and even at least 1250 MPa, total elongation TE, in accordance with ISO standard 6892-1 of at least 13% and even more than 14%, and HER expansion ratio in accordance with ISO standard 16630: 2009 of at least 30% and even at least 35 %.

The tensile yield stress YS may be more than 1000 MPa, especially in the case of a carbon redistribution time Pt of at least 110 seconds.

Example 1

For the purposes of the examples and comparisons, sheets were made from steel compositions according to Table I, with the element levels expressed on a weight basis. Table I lists transformation temperatures such as Ac1 and Ac3. Ac1 and Ac3 values were measured using dilatometry.

Table I

In this table, the designation "stop." speaks of an element present only as a remainder, and the absence of deliberate addition of this element.

The sheets were hot rolled, then coiled at 550 ° C.

The inventors first evaluated the effect of chamber furnace annealing on the properties of hot rolled sheet steels.

To this end, the inventors subjected a hot rolled steel sheet made of steel I1 to annealing in a chamber furnace at 550 ° C for 2 days and compared the mechanical properties of a chamber annealed steel sheet with the mechanical properties of the same hot rolled steel sheet, not subjected to annealing in a chamber furnace.

The mechanical properties, namely tensile yield strength YS, tensile strength TS, uniform elongation UE and total elongation TE are given in Table II below.

Table II

Example Annealing in a chamber furnace? YS (MPa) TS (MPa) UE (%) TE (%) 1 Not 798 1145 7.4 ten 2 Yes 597 995 9.4 13.8

As shown by these results, annealing in a chamber furnace results in softening of the hot-rolled steel sheet, thereby improving its cold rolling properties.

In addition, hot rolled steel sheets made from steels I1 and R1 were annealed in a chamber furnace for 2 days at 550 ° C., pickled and cold rolled. Cold rolled sheets were subjected to annealing, quenching and carbon redistribution. After redistribution of carbon, the sheets were coated by hot-dip galvanizing at 460 ° C., followed by cooling to room temperature.

Processing conditions are shown in Table III.

Table III

In this table, Ms denotes the onset temperature of martensitic transformation for austenite resulting from annealing, T _A is the annealing temperature, t _A is the annealing time, Vc is the average cooling rate between the annealing temperature T _A and the quenching temperature, QT is the temperature quenching, PT is the temperature of carbon redistribution, and Pt is the time of carbon redistribution.

The microstructures obtained for the sheets are shown in Table IV. F indicates the surface fractional concentration of intercritical ferrite, M indicates the surface fractional concentration of martensite, PM indicates the level of the percentage of martensite undergoing redistribution of carbon in martensite, RA indicates the surface fractional concentration of retained austenite, and B indicates the surface fractional concentration of lower bainite. For each steel sheet, Table IV shows whether the proportion or percentage level of each microstructure component is within the target range.

In addition, the expansion ratio HER measured in accordance with standard ISO 16630: 2009, the tensile yield strength YS, the tensile strength TS, the uniform elongation UE and the total elongation TE were determined for each steel sheet. Tensile yield YS, tensile strength TS, uniform elongation UE and total elongation TE were measured in accordance with ISO standard ISO 6892-1, published in October 2009. Properties are listed in Table V.

In this table, “n / a” denotes that the property was not defined.

As shown by these examples, coated steel sheets having a tensile strength of at least 1180 MPa and a total elongation TE according to ISO 6892-1 can be obtained by carrying out the process according to the invention. constituting at least 13%. These steel sheets also have a tensile yield strength of at least 800 MPa and a HER expansion ratio of at least 30% according to ISO 16630: 2009. These steels are also characterized by a uniform elongation of at least 9%, and generally more than 10%.

Table V

As shown by the comparison of examples 3 and 4, the target properties will be achieved only when the quenching temperature is in the range from Ms - 60 ° C to Ms - 10 ° C. In contrast, in the case of a quenching temperature QT lower than Ms - 60 ° C (example 3), the fractional concentration of martensite created after quenching would be excessively high such that a sufficient fractional concentration of austenite could not be achieved. As a result, a total elongation of at least 13% is not achieved.

As shown by the comparison of examples 4 and 5, the target properties will be achieved only in the case of the carbon redistribution temperature PT, comprised in the range from 410 ° C to 470 ° C. In contrast, if the carbon redistribution temperature PT is less than 410 ° C (Example 5), there will be insufficient carbon redistribution from martensite to austenite such that austenite will not stabilize sufficiently to ensure that there is a total elongation of at least 13%.

As shown by comparison of Examples 6 and 7, lowering the annealing temperature from 900 ° C to 880 ° C makes it possible to achieve an improved balance between tensile strength and total elongation.

As examples 6, 8 and 9 demonstrate, the method is very reliable with variations in the annealing time t _A and the carbon redistribution time Pt, which can be caused by variations in the production line. Therefore, the achievement of the desired mechanical properties is achieved using the method of the invention, regardless of undesirable variations in the production line. However, as it should be noted, longer times of redistribution of carbon Pt lead to an increase in the yield strength in tensile YS (example 10) due to a decrease in the fractional concentration of fresh martensite.

Using examples 10 to 12, the inventors investigated the effect of the cooling rate Vc during quenching on mechanical properties.

As shown by these examples, in the case of a cooling rate of more than 30 ° C / s, the target properties will be achieved. In contrast, in the case of a cooling rate of 30 ° C / s or less (Example 13), after cooling, more than 5% ferrite will be generated, and an insufficient fractional concentration of martensite will be obtained at QT. As a consequence, the final structure is characterized by an insufficient level of the percentage of martensite subjected to redistribution of carbon, and contains converted ferrite. Therefore, a tensile yield strength of at least 800 MPa and a tensile strength of at least 1180 MPa are not achieved.

In addition, as shown in examples 10 and 11, in the case of a cooling rate of at least 50 ° C / s, a total elongation of more than 14% will be achieved, and in the case of a cooling rate Vc of at least 60 ° C / s, a tensile strength of at least 1250 MPa and an overall elongation of at least 14% will be achieved.

As Example 13 demonstrates, in the case of excessively low annealing, quenching and carbon redistribution temperatures, the desired properties will not be achieved. In particular, due to the low annealing temperature, the fractional concentration of intercritical ferrite is more than 10%. In addition, due to low quenching temperatures and carbon redistribution, the fractional concentration of martensite created after cooling is excessively high, and during carbon redistribution, insufficient carbon redistribution occurs from this martensite to austenite. As a result, a tensile strength of at least 1180 MPa and a total elongation of at least 13% are not achieved.

Examples 14 and 15 were produced from steel R1, which is characterized in particular by an insufficient Al content.

In addition, examples 14 and 14 were produced at excessively low annealing and quenching temperatures.

Thus, examples 14 and 15 have a total elongation of less than 13% and a widening ratio of less than 30%.

The accompanying figure is a photomicrograph showing the microstructure of Example 11. In this figure, F is intercritical ferrite, B is lower bainite, PM is carbon redistributed martensite, FM is fresh martensite and RA is retained austenite.

Example 2

Sheets were made from steel compositions according to Table VI, with the element levels expressed on a weight basis. Table I lists transformation temperatures such as Ac1 and Ac3. Ac1 and Ac3 values were measured using dilatometry. In table VI, steel I1 is the same steel as in table I.

Table VI

In this table, the designation "stop." speaks of an element present only as a remainder, and the absence of deliberate addition of this element.

The sheets were hot rolled, then coiled at 550 ° C. Hot rolled steel sheets made from steels I1 and R1 were annealed in a chamber furnace for 2 days at 550 ° C., pickled and cold rolled to a thickness of 1.6 mm.

Cold rolled sheets were subjected to annealing, quenching and carbon redistribution. After redistribution of carbon, the sheets were coated by hot-dip galvanizing at 460 ° C., followed by cooling to room temperature.

The processing conditions are shown in Table VII.

Table VII

In this table, Ms denotes the start temperature of martensitic transformation for austenite resulting from annealing, T _A is the annealing temperature, t _A is the annealing time, Vc is the average cooling rate between the annealing temperature T _A and the quenching temperature, QT is the temperature quenching, PT is the temperature of carbon redistribution, and Pt is the time of carbon redistribution. Example 16 corresponds to Example 6 above.

The microstructures obtained for the sheets are shown in Table VIII. F indicates the surface fractional concentration of intercritical ferrite, M indicates the surface fractional concentration of martensite, PM indicates the level of the percentage of martensite undergoing redistribution of carbon in martensite, RA indicates the surface fractional concentration of retained austenite, and B indicates the surface fractional concentration of lower bainite.

Table VIII

In addition, the expansion ratio HER measured in accordance with standard ISO 16630: 2009, the tensile yield strength YS, the tensile strength TS, the uniform elongation UE and the total elongation TE were determined for each steel sheet. Tensile yield YS, tensile strength TS, uniform elongation UE and total elongation TE were measured in accordance with ISO standard ISO 6892-1, published in October 2009. Properties are listed in Table IX.

In this table, “n / a” denotes that the property was not defined.

Table IX

Example Steel YS (MPa) TS (MPa) UE (%) TE (%) HER (%) sixteen I1 970 1196 11.1 15.2 32 17 R2 900 1210 but 15 40

Sheets made from steels I1 or R2 (examples 16 and 17) were welded using resistance spot welding using alternating electric current at 60 Hz and an electrode force of 4.5 kN under various conditions according to Table X. The electrodes are located perpendicular sheet steel.

- By applying different intensities, it is possible to establish a suitable welding range, defined by the Imin value, which is the minimum intensity above which contraction fracture is observed during the resistance spot weld tensile shear test, and the value of Imax, which is the intensity at which the displacement of liquid metal begins to be observed in resistance spot welding. The choice of intensity in industrial settings is often made in the region of this latter value, since this corresponds to a large diameter of the core of the weld spot, which makes it possible to obtain high tensile characteristics of the weld. In the present case, welding was performed at I _max and slightly higher in the displacement region, that is, I _max + 10%. Despite an increase in the susceptibility to embrittlement of LMOs as a result of welding at an intensity ranging from I _max to I _max + 10%, this condition in some cases can occur in industrial practice.

Welding parameters are:

- diameter of the working end of the electrode: 6 mm,

- welding force: 4.5 kN,

- welding time: 380 ms,

- cooling time: 0 ms,

- holding time: 300 ms.

- As a result of welding two or three sheets together and creating a stack configuration, the susceptibility to cracking due to embrittlement of LMOs is greater with increased stack thickness. In particular, for the production of double-layer bags, sheets 16 and 17 are welded with galvanized steel DP980, characterized by the following composition: 0.1C - 2.2Mn - 0.3Mo - 0.2Cr - 0.01 Nb - 0.03Ti - 0.001 B and having a thickness of 1.6 mm. For the production of three-layer bags, sheets 16 and 17 were welded to two galvanized sheets made of extra deep drawing steel (having a tensile strength of 270 MPa), each having a thickness of 1.5 mm. These other steels are selected because their resistance spot welding requires a higher level of electric current to obtain proper welds compared to the steels of the invention having a tensile strength of more than 1180 MPa. This high level of electric current induces a large heat input and, as a consequence, induces the appearance of more cracks due to embrittlement of LMOs during welding of high strength steels. Thus, the severity of the welding conditions increases. Table X shows the cumulative bag thickness. In these packages, welding is carried out in such a way that the steel sheet having a tensile strength of more than 1180 MPa (the steel sheets of examples 16 or 17) has one surface in contact with the welding electrode. Possible cracks are more susceptible to formation in the indentation zone created by the welding electrode on the surface of the sheet.

Table X

Welding conditions Number of layers in a package Bag thickness (mm) Welding intensity a 2 I _max and I _max + 10% b 3 I _max and I _max + 10%

The observation and quantification of cracks due to embrittlement of LMOs was carried out under the following conditions: after transverse semi-cutting and fine polishing, 20 welds from resistance spot welding, ten of which are welded using I = I _max , and ten others are welded using I = I _max + 10%, the sections of welds were observed using an optical microscope with a magnification in the range from 10 to 1000, for example, 200. For each weld from resistance spot welding, the number of cracks having a depth of more than 100 microns was measured, including representatives of each series of 10 welds from spot resistance welding. In addition, for each series of 10 resistance spot welds produced under the same conditions, a maximum crack size of more than 100 µm was determined.

For packages of two layers, high resistance to cracking due to embrittlement of LMOs will be obtained in the case of an average number of cracks having a depth of more than 100 microns, which is less than 0.1, in the case of welding at I = I _max or I _max + 10%.

For packages of three layers, high resistance to cracking due to embrittlement of LMOs will be obtained in the case of an average number of cracks having a depth of more than 100 microns, which is less than 2, in the case of welding at I = _Imax or in the case of an average amount cracks of less than 4, in the case of welding at I _max + 10%.

Table XI shows the intensity Imax, the average number of LMO embrittlement cracks as determined under welding conditions at I _max or I _max + 10%, and the maximum crack size having a size greater than 100 μm.

Table XI

In this table, 16a denotes 16 steel sheets welded under conditions a. The same applies to example 17a (sheet 17 welded under conditions a) and by analogy to examples 16b and 17b.

With regard to the weldability using resistance spot welding, and in accordance with the demonstration in the above example 2, the sheets according to the invention are characterized by a low susceptibility to embrittlement of LMOs. This means that with such steels it is possible to produce structures that include resistance spot welded seams, such as car bodies, for which the likelihood of the number of cracks in the resistance spot welded seams is such that the average is less than than 6 cracks per resistance spot weld, and the probability of less than 10 cracks is 98%.

In addition, according to the demonstration in the above example, the number of cracks having a size greater than 100 microns is greatly reduced as compared to that of the state of the art sheet steels.

In particular, a welded structure including a resistance spot weld from at least two steel plates can be produced by producing the first steel plate using the method according to the invention, for example, by applying a Zn or Zn alloy coating, obtaining a second steel sheet characterized by a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%, and resistance spot welding for the first steel sheet and the second steel sheet. The second steel sheet can, for example, be produced using the method according to the invention and can be coated with Zn or a Zn alloy.

- Thus, a welded structure is obtained, characterized by a low susceptibility to embrittlement of LMOs. For example, for such a welded structure including at least ten resistance spot welds, the average number of cracks per resistance spot weld is less than 6.

For example, a welded structure can be produced by producing a first steel plate and a second steel plate, overlapping the first steel plate and the second steel plate, applying a force ranging from 3.5 to 5 kN, using electrodes located perpendicular to the superposed plates, and resistance spot welding for the first and second plate steels in order to produce at least 10 resistance spot welds, the intensity being comprised in the range from Imax to 1.1 * Imax, with Imax being the intensity when which begins to see the displacement of liquid metal by resistance spot welding for said first coated steel sheet and said second steel sheet.

Especially in the case of a welded structure, which is a welded structure of only two sheets (first and second sheet steels), even when welding sheet steels under severe conditions, especially when using intensities ranging from I _max to I _max + 10 %, the average number of cracks in resistance spot welds having a depth greater than 100 microns is less than 0.1. In general, of the ten resistance spot welds, none include cracks greater than 100 microns in depth.

In the case of a welded structure, which is a welded structure of three sheets, the third sheet is characterized by a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%, even with welding of sheet steel under severe conditions, especially when using intensities ranging from I _max to I _max + 10%, the average number of cracks in resistance spot welds having a depth of more than 100 microns is less than 4. In particular, when the intensity is equal to the value of Imax, the average number of cracks having a depth of more than 100 microns is less than 2.

Sheet steels, optionally welded using resistance spot welding in accordance with the invention, are advantageously used in the manufacture of structural parts for motor vehicles because they exhibit high deformability during the manufacturing process and high energy absorption in the event of a collision. Resistance spot welds according to the invention are also advantageously used for the fabrication of structural parts of motor vehicles, since the possible initiation and propagation of cracks located in the welded zones is greatly reduced.

Claims (97)

1. A method of manufacturing a coated steel sheet having a tensile yield strength of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 1 in accordance with ISO 6892-1 at least 13%, and a HER hole expansion ratio of at least 30% according to ISO 16630: 2009, the method includes the following sequential steps: obtaining cold-rolled sheet steel made of steel characterized by a chemical composition containing, in wt%: 0.15 ≤ C ≤ 0.23, 1.4 ≤ Mn ≤ 2.6, 0.6 ≤ Si ≤ 1.3, while C + Si / 10 ≤ 0.30, 0.4 ≤ Al ≤ 1.0, and Al ≥ 6 (C + Mn / 10) - 2.5, 0.010 ≤ Nb ≤ 0.035, 0.1 ≤ Mo ≤ 0.5, Fe and inevitable impurities - the rest, annealing the cold rolled steel sheet at an annealing temperature T _A comprised between 860 ° C and 900 ° C, while the annealing temperature is less than Ac _3, to obtain an annealed steel sheet having a structure of at least 90% austenite and at least 2% intercritical ferrite, cooling the annealed steel sheet from the annealing temperature T _A up to the cooling temperature QT between Ms - 10 ° C and Ms - 60 ° C at a cooling rate Vc of more than 30 ° C / s to obtain a hardened sheet, heating of the hardened sheet from the cooling temperature QT to the temperature PT, between 410 ° C and 470 ° C, and holding the sheet at this temperature PT for the time Pt, between 60 and 130 s, coating sheet steel by immersion in a molten bath, cooling the steel sheet to room temperature to obtain a coated steel sheet having a microstructure comprising, expressed in area fractions: from 45% to 68% martensite, while the martensite consists of martensite, in which the redistribution of carbon has occurred, and fresh martensite, and from 85% to 95% of martensite is martensite, in which the redistribution of carbon has occurred, while the mentioned martensite, in which redistribution of carbon has occurred, characterized by a C content of at most 0.45%, from 10% to 15% of retained austenite, from 2% to 10% intercritical ferrite, 20% to 30% lower bainite. 2. The method according to claim 1, in which the hardened sheet immediately before heating to the temperature RT has a structure that, when expressed in area fractions, consists of: from 2% to 10% intercritical ferrite, at least 10% retained austenite, at least 38% martensite and at least 15% lower bainite, at most 5% ferrite converted. 3. A method according to claim 1, wherein the step of producing the cold rolled steel sheet comprises: hot rolling of a semi-finished product made of said steel to obtain hot rolled steel sheet, rolling said hot-rolled steel sheet into a coil at a temperature Tc comprised in the range from 400 ° C to 750 ° C, carrying out annealing in a chamber furnace at a temperature T _HBA , enclosed in the range from 500 ° C to 700 ° C, for a period of time enclosed in the range from 2 to 6 days, cold rolling said hot rolled steel sheet to obtain said cold rolled steel sheet. 4. The method according to claim 1, wherein the cold-rolled steel sheet is kept at an annealing temperature T _A for an annealing time t _A comprised between 80 and 180 seconds. 5. The method according to any one of claims. 1-4, in which the annealing temperature T _A is at most 880 ° C to obtain an annealed steel sheet having a structure of at least 90% austenite and more than 5% intercritical ferrite, and the microstructure of the coated steel sheet contains more than 5 % intercritical ferrite. 6. The method according to any one of claims. 1-4, in which the cooling of the annealed steel sheet from the annealing temperature T _A to the cooling temperature QT is carried out at an average cooling rate Vc of at least 50 ° C / s, and the microstructure of the coated steel sheet consists in terms of area fractions of : from 45% to 68% martensite, while the martensite consists of martensite, in which the redistribution of carbon has occurred, and fresh martensite, and from 85% to 95% of martensite is martensite, in which the redistribution of carbon has occurred, while the mentioned martensite, in which redistribution of carbon has occurred, characterized by a C content of at most 0.45%, from 10% to 15% of retained austenite, from 2% to 10% intercritical ferrite, from 20% to 30% lower bainite, the coated steel sheet having a total elongation of at least 14%. 7. The method according to claim 6, wherein the cooling of the annealed steel sheet from the annealing temperature T _A to the cooling temperature QT is carried out at an average cooling rate Vc of at least 60 ° C / s, the total elongation being at least 14% and the tensile strength is at least 1250 MPa. 8. The method according to any one of claims. 1-4, in which 0.6% ≤ Si <1.0% and 0.7% ≤ Al ≤ 1.0%. 9. The method according to any one of claims. 1-4, in which 0.17% ≤ C ≤ 0.21%. 10. The method according to any one of claims. 1-4, in which 1.9% ≤ Mn ≤ 2.3%. 11. The method according to any one of claims. 1-4, in which the steel sheet is coated with a Zn or Zn alloy. 12. A method of manufacturing a welded structure from at least two sheet steels joined by at least one resistance spot welded seam, comprising: obtaining the first coated steel sheet by the method according to any one of paragraphs. 1-4, provision of a second sheet steel with a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%, joining said first coated steel sheet and the second steel sheet by means of resistance spot welding. 13. Coated sheet steel obtained from steel having the following chemical composition, in wt%: 0.15 ≤ C ≤ 0.23, 1.4 ≤ Mn ≤ 2.6, 0.6 ≤ Si ≤ 1.3, while C + Si / 10 ≤ 0.30, 0.4% ≤ Al ≤ 1.0, and Al ≥ 6 (C + Mn / 10) - 2.5, 0.010 ≤ Nb ≤ 0.035, 0.1 ≤ Mo ≤ 0.5, Fe and inevitable impurities - the rest, wherein said coated steel sheet has a microstructure comprising, when expressed in area fractions: from 45% to 68% martensite, while the martensite consists of martensite, in which the redistribution of carbon has occurred, and fresh martensite, and from 85% to 95% of martensite is martensite, in which the redistribution of carbon has occurred, while the mentioned martensite, in which redistribution of carbon has occurred, characterized by a C content of at most 0.45%, from 10% to 15% of retained austenite, from 2% to 10% intercritical ferrite, 20% to 30% lower bainite. 14. The sheet steel of claim 13, wherein the retained austenite has an average C content ranging from 1.0% to 1.3%. 15. The steel sheet of claim 13, wherein the microstructure of the coated steel sheet contains more than 5% intercritical ferrite. 16. Sheet steel according to any one of paragraphs. 13-15, in which 0.6% ≤ Si <1.0% and 0.7% ≤ Al ≤ 1.0%. 17. Sheet steel according to any one of paragraphs. 13-15, in which 0.17% ≤ C ≤ 0.21%. 18. Sheet steel according to any one of paragraphs. 13-15, in which 1.9% ≤ Mn ≤ 2.3%. 19. Sheet steel according to any one of paragraphs. 13-15, characterized by a tensile yield point of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 13% in accordance with ISO 6892-1, and a coefficient hole spreading, according to ISO 16630: 2009 HER, at least 30%. 20. Sheet steel according to any one of paragraphs. 13-15 having a Zn or Zn alloy coating, wherein the coating is applied at a temperature of less than 480 ° C. 21. Sheet steel according to any one of paragraphs. 13-15, in which the microstructure consists of: from 45% to 68% martensite, while the martensite consists of martensite, in which the redistribution of carbon has occurred, and fresh martensite, and from 85% to 95% of martensite is martensite, in which the redistribution of carbon has occurred, while the mentioned martensite, in which redistribution of carbon has occurred, characterized by a C content of at most 0.45%, from 10% to 15% of retained austenite, from 2% to 10% intercritical ferrite, from 20% to 30% lower bainite, at most 5% ferrite converted. 22. Sheet steel according to any one of paragraphs. 13-15, in which the microstructure consists of: from 45% to 68% martensite, while the martensite consists of martensite, in which the redistribution of carbon has occurred, and fresh martensite, and from 85% to 95% of martensite is martensite, in which the redistribution of carbon has occurred, while the mentioned martensite, in which redistribution of carbon has occurred, characterized by a C content of at most 0.45%, from 10% to 15% of retained austenite, from 2% to 10% intercritical ferrite, 20% to 30% lower bainite. 23. A welded structure of sheet steel, containing at least two sheet steel, in which the first sheet steel is a coated steel sheet according to any one of paragraphs. 13-15, and the second sheet steel is characterized by a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%, while the two sheet steels are joined by ten welds , obtained by resistance spot welding, while the average number of cracks per one weld, obtained by resistance spot welding, is less than 6. 24. A welded structure according to claim 23, in which the second steel sheet is a coated steel sheet according to any one of claims. 13-15. 25. A welded structure according to claim 23, wherein the welded structure is a welded structure of two sheets consisting of said first steel sheet and said second steel sheet, and the average number of cracks having a depth of more than 100 microns, including at least at least ten of the specified welds is less than 0.1. 26. A welded structure according to claim 23, wherein the welded structure is a welded structure from said first steel sheet, said second steel sheet, and a third steel sheet having a composition such that C + Si / 10 ≤ 0.30% and Al ≥ 6 (C + Mn / 10) - 2.5%, and the average number of cracks having a depth of more than 100 microns, among at least ten of these welds is less than 4. 27. A method of manufacturing a welded structure from sheet steel according to claim 25, including: providing said first steel sheet and said second steel sheet, partial overlap of said first sheet steel and second sheet steel, application of a force within the range of 3.5 to 5 kN when using electrodes located perpendicular to the superimposed sheets, resistance spot welding of said first and second sheet steels to produce at least 10 resistance spot welds at an intensity ranging from Imax to 1.1 * Imax, where Imax is the rate at which liquid metal displacement begins to occur in resistance spot welding of said first coated steel sheet and the second steel sheet. 28. The use of coated steel sheet obtained by the method according to any one of paragraphs. 1-4, for the manufacture of structural parts for motor vehicles. 29. The use of coated steel sheet according to any one of paragraphs. 13-15 for the manufacture of structural parts for motor vehicles. 30. The use of a welded structure of sheet steel, obtained by the method according to claim 12, for the manufacture of structural parts for motor vehicles. 31. Application of a welded structure made of sheet steel according to claim 23 for the manufacture of structural parts for motor vehicles.

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